org.jsimpledb.kv.mvcc.SnapshotRefs Maven / Gradle / Ivy

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JSimpleDB classes relating to the key/value store API.

There is a newer version: 3.6.1


/*
 * Copyright (C) 2015 Archie L. Cobbs. All rights reserved.
 */

package org.jsimpledb.kv.mvcc;

import com.google.common.base.Preconditions;

import java.io.Closeable;

import org.jsimpledb.kv.CloseableKVStore;
import org.jsimpledb.kv.KVStore;
import org.slf4j.LoggerFactory;

/**
 * Holds a count of references to a {@link CloseableKVStore} and actually {@link CloseableKVStore#close close()}'s it
 * when the reference count reaches zero.
 *
 * 
 * Instances are thread safe.
 */
public class SnapshotRefs {

    private final CloseableKVStore snapshot;
    private int refs;

    /**
     * Constructor.
     *
     * 
     * Initially the reference count is set to one.
     *
     * @param snapshot database snapshot
     */
    public SnapshotRefs(CloseableKVStore snapshot) {
        Preconditions.checkArgument(snapshot != null, "null snapshot");
        this.snapshot = snapshot;
        this.refs = 1;
        synchronized (this) { }
    }

    public synchronized KVStore getKVStore() {
        Preconditions.checkState(this.refs > 0, "no longer referenced");
        return this.snapshot;
    }

    public synchronized void ref() {
        Preconditions.checkState(this.refs > 0, "no longer referenced");
        this.refs++;
    }

    public synchronized void unref() {
        Preconditions.checkState(this.refs > 0, "no longer referenced");
        if (--this.refs == 0)
            this.snapshot.close();
    }

    /**
     * Create an empty {@link Closeable} that, when {@link Closeable#close close()}'d, decrements the reference count.
     *
     * @return unreferencing {@link Closeable}
     */
    public Closeable getUnrefCloseable() {
        return new Closeable() {
            private boolean closed;
            @Override
            public synchronized void close() {
                if (this.closed)
                    return;
                SnapshotRefs.this.unref();
                this.closed = true;
            }
        };
    }

    @Override
    protected void finalize() throws Throwable {
        try {
            if (this.refs > 0) {
                LoggerFactory.getLogger(this.getClass()).warn(this + " leaked with " + this.refs + " remaining references");
                this.refs = 0;
                this.snapshot.close();
            }
        } finally {
            super.finalize();
        }
    }
}